The present invention has been developed to overcome problems associated with the transportation of oil sands from a mine site to a hydrocarbon extraction plant. The oil sand is transported by an endless belt conveyor system.
In the commercial open-pit oil sand-mining operation of the present assignees, carried out in the Athabasca region of Northern Canada, about 300,000 tonnes per day of oil sand is mined. The mined sands are conveyed to a distant extraction plant where bitumen is recovered therefrom using the hot water extraction process. Typically, the oil sand must be transported over distances ranging up to 5.5 km.
As-mined oil sand comprises bitumen, water, quartz sand and clays. Oil sand `ore` is classified as rich, average or lean depending upon its bitumen content. An average ore analysis could be 84 wt. % sand; 11 wt. % bitumen; and 5 wt. % water. Oil sand will vary in its physical properties depending upon its ore grade.
In its physical characteristics, oil sand is both abrasive and cohesive due to its quartz and bitumen content respectively. As it dries, oil sand, because of its clay content, hardens but continues to remain adhesive. Additionally, external factors such as fluctuations in ambient temperatures, or density variations due to snow or rainfall pick-up will serve to further alter the adhesivity of the oil sand.
The oil sand is deposited onto the conveyor train as an unevenly distributed load. Under its own weight, and as a consequence of vibration in the conveyor system, the underlying oil sand layer immediately adjacent the belt becomes compacted. The hardened oil sand in this layer builds up into a consolidated mass of several centimeters thickness. After the load is discharged, this layer remains on the belt. The layer becomes further compacted as the belt travels over the tensioning snub pulley of the conveyor under compressive stress. This hardened, compacted, adherent oil sand layer is hereinafter referred to as the `oil sand layer`.
In addition to the adhesive layer on the load-carrying surface of the belt, some oil sand also accumulates as a similar layer on the underside surface of the belt loop. This is oil sand which has dropped onto the underside surface on the return leg.
Typically, the oils and layer will build up until it reaches a thickness of about 1/2". Then, what is referred to as `peeling` from the downwardly facing surface of the belt will take place; more particularly, one oil sand layer will separate or break away from another, with the `peel` falling onto the ground beneath. Because peeling does not take place until the layer reaches a considerable thickness, it will be readily appreciated that the weight of the build-up prior to peeling significantly and deleteriously affects conveyor power draw requirements. Furthermore, serious removal problems are posed by the volume of deposited peel which has to be removed from beneath the conveyor belts. For example, at the present assignees' commercial plant described supra, four front-end loaders equipped with power rakes are employed around the clock to remove peel. It will be noted too that, when using these rakes, it is not uncommon for the conveyor belt support frames to experience damage.
During the summer months, the adhesivity of the oil sand layer to the belt increases and the problem becomes more severe.
It is undesirable for the belt to be continuously carrying dead tonnage. In addition, the carrying of this dead weight can be responsible for untracking of the belt.
At present, the prior art approaches to solving this problem have focused either on prevention of build-up of oil sand on the belt, by an application of diesel oil to the surface thereof, or jetting water (or water/glycol, in the winter months) against the layer, to remove it. The disadvantages inherent with the first solution are that the diesel oil is expensive and destructive of the belts. It also creates a fire hazard problem. The latter solution is not only ineffective but is also expensive, involving the expenditure of several million dollars per annum on glycol alone.
There exists, therefore, a need for a better method for removing the compacted oil sand layer from the belt. It is desirable that the method eliminate the requirement for the application of costly solutions, be effective on an all-year-round basis, and be inexpensive.
In the prior art, mechanical devices have been suggested for cleaning conveyor belts. See for example those disclosed in U.S. Pat. Nos. 3,047,133 and 3,430,758, issued to A. Searles, and 3,540,573, issued to J. K. Keim. However, these prior art devices would not be effective to handle oil sand. They would either wear out rapidly because of the abrasive nature of the oil sand or would become clogged by it.